This invention relates to the preparation of diaryl carbonates by carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply "carbonylation" for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a heavy Group VIII metal; i.e., a Group VIII metal having an atomic number of at least 44, said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
The production of carbonates may be improved by including a metal-based cocatalyst along with the heavy Group VIII metal catalyst. Although the identity of suitable metal-based cocatalysts will depend on specific reaction conditions including the identity of reactants and other members of the catalyst package, some general guidance can be found in U.S. Pat. Nos. 4,187,242 and 4,201,721.
A further development in the carbonylation reaction, including the use of specific lead compounds as cocatalysts, is disclosed in U.S. Pat. No. 5,498,789. Also required according to that patent is the use of quaternary ammonium or phosphonium halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package. Compounds characterized as inert solvents, such as toluene, diethyl ether, diphenyl ether and acetonitrile, can also be present.
The commercial viability of the carbonylation reaction would be greatly increased if a less expensive compound could be substituted for the quaternary ammonium or phosphonium halide. Substitution of such compounds as sodium bromide, however, result in the isolation of the desired diaryl carbonate in low or insignificant yield.
It is of interest, therefore, to develop catalyst systems which include an inexpensive halide compound and which can efficiently produce diaryl carbonates. Some such systems are known. Reference is made, for example, to Japanese Kokai 10/316,627, which discloses the use of palladium and a lead or manganese compound in combination with a halide such as sodium bromide and with an amide or alkylurea. U.S. Pat. No. 5,726,340 and Japanese Kokai 9/278,716 disclose similar systems in which the lead is combined with another metal and in which inert solvents such as those mentioned hereinabove may be present. The development of other systems employing relatively inexpensive halides, however, remains desirable.